This invention relates to a method for producing styrenic block copolymers wherein living block copolymer chains are linked together by utilizing a linking compound which has at least two functional groups. More particularly, the present invention relates to a method for achieving high efficiency coupling of styrenic block copolymers by using 4-vinyl-1-cyclohexene diepoxide as the linking compound to couple two living block copolymer chains together.
The production of styrene-diene-styrene triblock copolymers by coupling of the styrene-diene diblock is well known and has a number of advantages over the production of corresponding polymers by fully sequential polymerization. Higher overall rates are achieved and more symmetrical styrene blocks are formed since the coupling process avoids the crossover problem encountered when initiating a third styrene block in the fully sequential polymerization method. Styrene block symmetry is important to a number of physical properties including tensile strength.
A large number of organic compounds have been reported as useful as coupling agents for such block copolymers. The most commonly used coupling agent for the production of linear polymers is dibromoethane. Coupling efficiencies in the 80 to 85 percent range can easily be achieved with dibromoethane and the products have excellent melt viscosity stability. However, these products discolor after aging at elevated temperature due to the presence of lithium bromide. Other coupling agents, including methyl formate and a variety of silanes, are known to produce color stable linear polymers with reasonably high coupling efficiency but most of the products exhibit significantly lower melt viscosity stability.
U.S. Pat. No. 5,461,095 describes coupled polymers which are said to have a high coupling efficiency, good melt stability, and also good color stability. These polymers are said to have been produced by using aromatic epoxy compounds such as high purity, high diepoxy content, diglycidyl ether of bisphenol A (DGEBA) epoxy resins as the coupling agent. EPON(copyright) 825 resin from Shell Chemical Company is one example of this class of resins. Unfortunately, these aromatic epoxies have very low solubility in the aliphatic hydrocarbon solvents generally used for anionic polymerization of styrene-diene block copolymers, such as cyclohexane. One consequence of this is that efficient coupling is hard to achieve without vigorous mixing. These aromatic epoxy resins also tend to be quite viscous, making precise metering, which is a requirement for high coupling efficiency, more difficult and, since the solubility is low, dilution with a process solvent is not an option.
WO 99/01490 describes the use of glycidyl ethers of aliphatic polyalcohols as coupling agents. While these materials have a lower viscosity and good solubility in hydrocarbon solvents, commercially available grades are contaminated with high levels of monoepoxy and hydroxylic material. Purification by an expensive vacuum distillation is required before high coupling efficiency can be achieved.
4-vinyl-1-cyclohexene diepoxide (VCHD) is a low viscosity, hydrocarbon soluble diepoxy compound that is commercially available in very high purity (high diepoxy content). However, it has generally been considered too unreactive to be of use in the production of linear comparative test V6,4-vinyl-1-cyclohexene diepoxide was used to couple a living polybutadiene polymer at 50xc2x0 C. The coupling yield was only 50.7%. In most applications, it is desirable for the coupling efficiency to be at least 70%, preferably close to 80%.
It can be seen that there is a need for a coupling agent for styrene-diene block copolymers which will give a high coupling efficiency and also produce products which are highly melt stable and have good color stability, and also exhibits relatively good solubility in aliphatic hydrocarbon solvents. The present invention provides such a coupling agent and a process for achieving high coupling efficiency.
In one aspect, the present invention is a process for coupling styrenic block copolymers comprising admixing vinyl cyclohexene dioxide with a living block copolymer comprising at least one block of polymerized vinyl aromatic hydrocarbon monomers and at least one block of polymerized conjugated diene monomers, at a molar ratio of from 0.40 to 0.60 and carrying out a coupling reaction at a temperature of at least 65xc2x0 C. to produce a coupled styrenic block copolymer.
In another aspect, the present invention is a polymer composition comprising at most 30% by weight of a styrenic block copolymer comprising at least one block A of polymerized vinyl aromatic monomer(s) and at least one block B of polymerized conjugated diene(s) and at least 70% by weight of the styrenic block copolymer coupled with vinyl cyclohexene dioxide.
This invention is a process for coupling styrenic block copolymers utilizing 4-vinyl-1-cyclohexene diepoxide (VCHD) as the coupling agent. First, styrene or another vinyl aromatic hydrocarbon is anionically polymerized to produce a living styrene polymer block of desired molecular weight. Next, a diene, such as butadiene or isoprene, is anionically polymerized onto the living end of the styrene polymer block. This polymerization is carried out such that the diene block has a molecular weight of one-half of the desired molecular weight of the final polymer. Next, 4-vinyl-1-cyclohexene diepoxide is added to the reaction mixture at a mole ratio of from 0.40 to 0.6, preferably 0.5 to 0.55, moles of 4-vinyl-1-cyclohexene diepoxide per mole of polymer, and the reaction is carried out at a temperature of at least 65xc2x0 C., preferably 75xc2x0 C. to 95xc2x0 C., for at least 15 minutes, preferably at least 30 minutes.
It would be desirable in the art of preparing coupled styrene-diene block copolymers to prepare such copolymers using a process which has a high coupling efficiency. It would also be desirable in the art of preparing such copolymers if the copolymers could be used as or to prepare pressure sensitive adhesives and hot melt adhesives.
As is well known, polymers containing both aromatic and ethylenic unsaturation can be prepared by copolymerizing one or more diolefins, particularly a diolefin, such as butadiene or isoprene, with one or more alkenyl aromatic hydrocarbon monomers, such as styrene. The copolymers may, of course, be random, tapered, block or a combination of these, in this case block. The products of this invention are preferably mixtures of at least 70% A-B-A triblocks, the remainder being A-B diblock, prepared by coupling A-B diblock copolymers, in which the terminal monomer unit of the B block is derived from isoprene or butadiene. The two resulting polymers may also be blended with other diblock or triblock polymers or with star or radial polymers, i.e. polymers of the general structure (A-B)n-X, where n is greater than 2. Mixed polymer structures can also be made by coupling with a mixture of 4-vinyl-1-cyclohexene diepoxide and other coupling agents.
Polymers of conjugated diolefins and copolymers of one or more conjugated diolefins and one or more alkenyl aromatic hydrocarbon monomers are frequently prepared in solution using anionic polymerization techniques. In general, when solution anionic techniques are used, these block copolymers are prepared by contacting the monomers to be polymerized simultaneously or sequentially with an organoalkali metal compound in a suitable solvent at a temperature within the range from about xe2x88x92150xc2x0 C. to about 300xc2x0 C., preferably at a temperature within the range from about 0xc2x0 C. to 100xc2x0 C. Particularly effective anionic polymerization initiators are organolithium compounds having the general formula:
RLin 
Wherein:
R is an aliphatic, cycloaliphatic, aromatic or alkyl-substituted aromatic hydrocarbon radical having from 1 to about 20 carbon atoms; and n is an integer of 1 to 4.
The molar ratio of the initiator to the monomer determines the block size, i.e. the higher the ratio of initiator to monomer the smaller the molecular weight of the block. For products in the molecular weight range of interest, this generally leads to an initiator concentration in the range of about 0.25 to about 50 millimoles per 100 grams of monomer. The monomer concentration (solids) is generally dictated by the limitations placed on the viscosity of the resulting polymer solution and heat removal during the polymerization. Polymerization solids are generally in the range of 5 percent by weight (% wt) and 50% wt, preferably, 20% wt and 40% wt.
In general, any of the solvents known in the prior art to be useful in the preparation of such polymers may be used. Suitable solvents, then, include straight- and branched-chain hydrocarbons such as pentane, hexane, heptane, octane and the like, as well as, alkyl-substituted derivatives thereof; cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane, cycloheptane and the like, as well as, alkyl-substituted derivatives thereof; aromatic and alkyl-substituted aromatic hydrocarbons such as benzene, naphthalene, toluene, zylene and the like; hydrogenated aromatic hydrocarbons such as tetralin, decalin and the like. Further, polar cosolvents may be used. Suitable cosolvents include linear and cyclic ethers such as methyl, ether, methyl ethyl ether, tetrahydrofuran and the like, and mixture of such ethers.
As described in U.S. Pat. No. 4,096,203 the disclosure of which is herein incorporated by reference, usually the styrene is contacted with the initiator. Next, the living polymer in solution is contacted with isoprene or another diene. The resulting living polymer has a simplified structure A-B-Li. It is at this point that the living polymer is coupled with a coupling agent.
In the prior art, such as that exemplified by U.S. Pat. Nos. 3,595,941 and 3,468,972, the disclosures of which are herein incorporated by reference, the effort was always made to select the particular coupling agent or reaction conditions that resulted in the highest coupling efficiency. High coupling efficiencies are desired herein as well in order to produce more A-B-A polymer. Coupling efficiency is defined as the number of molecules of coupled polymer divided by the number of molecules of coupled polymer plus the number of molecules of uncoupled polymer. In the present invention, the use of 4-vinyl-1-cyclohexene diepoxide as the coupling agent allows a coupling efficiency of at least 70 percent to be achieved.       #    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    molecules    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢                  (        SI        )            2            #    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    molecules    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢                  (        SI        )            2        ⁢          xe2x80x83        ⁢    plus    ⁢          xe2x80x83        ⁢    SI  
Coupling efficiency is usually determined by gel permeation chromatography (GPC).
It is not sufficient to achieve the desired result of the present invention to merely use 4-vinyl-1-cyclohexene diepoxide as the coupling agent. The coupling process must be carried out under specified conditions. First, the temperature of the coupling reaction must be maintained at at least 65xc2x0 C. and preferably at 75 to 95xc2x0 C. If the temperature is lower than 65xc2x0 C. then the coupling efficiency which can be achieved is much lower, i.e., such as the 50 percent coupling efficiency which was achieved in WO 99/01490. The temperature probably should not be higher than 95xc2x0 C. because thermal termination of some of the living polymer blocks may occur prior to coupling.
The coupling reaction is preferably carried out for at least 15 minutes and preferably at least 30 minutes. If the coupling reaction time is less than 15 minutes, then the overall coupling efficiency goal will not be achieved. It is probably not necessary to carry out the coupling reaction for more than 60 minutes because the small additional coupling that takes place is not justified by the cost of the increased time of reaction.
Finally, it is important to achieve the correct stoichiometry. Ideally, it would be expected that the maximum coupling efficiency would be achieved for a difunctional coupling agent when the molar ratio of the coupling agent to the polymeric lithium anions is exactly 0.5. In practice, there may be side reactions that not only limit the maximum achievable coupling, but also influence this ratio. In the case of 4-vinyl-1-cyclohexene diepoxide, it is preferable to err on the side of excess diepoxide. Coupling efficiencies of at least 70% can be achieved if this ratio is between 0.40 and 0.6; high coupling efficiencies are most reliably achieved when this ratio is between 0.5 and 0.55.
Following the coupling reaction or when the desired coupling efficiency has been obtained, the product is neutralized such as by the addition of terminators, e.g., hydrogen, water, alcohol or other reagents, for the purpose of terminating residual active anions. The product is then recovered such as by coagulation utilizing hot water or steam or both.
The coupled polymer is then finished and shipped off to the end user. The polymer is then combined with other components to form whatever end use composition is desired.